The sediments encountered by great water depths on the continental shelf of the Gulf of Guinea are composed of very fine particles and are characterised by very high water contents (w>100%), very high plasticity (PI > 80), relatively high sensitivity (3 <St <5) and strong potential in shear strength regain due to thixotropy.
Difficulties in determining the organic matter content, the grain size distribution and the mineralogy of these sediments by the standard methods routinely used by soil mechanics laboratories have been encountered and reported1.
Organic matter content can be determined by a number of methods, including loss of ignition, pyrolysis, dichromate oxidation, infrared spectroscopy and geochemical acidification. Grain size distributions can be obtained from sieving and hydrometer testing or by laser diffraction method, both techniques being associated with various dispersive agents. Mineralogical characterisation is usually carried out by X-ray diffraction (XRD), but quantitative results are often given without precisions on operating procedures and are considered as not significant by specialists.
The paper focuses on the determination of organic matter content, grain size distribution and mineralogical determination. Results obtained on typical Gulf of Guinea sediments using the various methods listed above are compared and discussed. Specific testing procedures are recommended for these sediments.
The sediments encountered at great water depths on the continental shelf of the Gulf of Guinea express the following general feature: very fine constituting particles, very high water contents (w> 100%), very high plasticity (PI > 80), relatively high sensitivity (3 <St <5) and strong potential in shear strength regain due to thixotropy. Such properties are usually characterised by geotechnical tests (index or mechanical tests) performed at a macroscopic scale without any reference to the microscopic aspects1. This paper highlights the benefit of combining both approaches for a better understanding of the physical and mechanical properties of the Gulf of Guinea deepwater soils.
The mechanical properties of a colloidal dispersion are essentially controlled by the particle size and shape distribution, and by the interparticle interactions (electrostatic interactions, bridging, friction, etc.). In soils and sediments, the individual mineral particles are generally assembled within aggregates of various size and tensile strength, according to the nature and surface properties of the particles, as well as the nature of the interparticle phase acting as a linking agent, such as organic matter, amorphous phases or ions. Better understanding of the mechanisms underlying the geomechanical properties of deep sea soils therefore requires thorough analysis of their micro-scale characteristics. These characteristics include the chemical nature and relative abundance of the constituting mineral and organic phases, which are indicative of generic size, shape, surface reactivity of the solid phases and of the nature of the interparticle interactions. Several methods and standardised procedures aimed at determining these characteristics are available. In the present work, three major aspects were investigated: the particle size distribution, the mineralogical determination and the organic carbon content.
The particle size distribution is an essential parameter in the geomechanical behaviour of soils and sediments.